Last month a campaign event in Chicago was cancelled amid fights between demonstrators and supporters. Days later, Mr Trump's Secret Service security detail had to leap to his aid when a protester tried to rush the podium at a rally in Dayton, Ohio.

And last year El Chapo, the Mexican drug lord, reportedly put a $100m bounty on Mr Trump's head.

Campaign insiders say he has been wearing the vest since at least February.

It claims Mr Trump came to Mr Ailes' aid in 2014 when the TV executive was embroiled in a law suit with his former PR adviser, Brian Lewis, whom he fired over an unflattering biography – written by the author of the magazine piece.

“When Roger was having problems, he didn’t call 97 people, he called me,” said Mr Trump.

Watch | Donald Trump's most outrageous quotes

02:11

Mr Lewis claimed to have information that would destroy Mr Ailes and his TV network. Mr Trump, who knew the lawyer retained by Mr Lewis, helped broker a pay-off, according to the magazine, but in so doing learned all about the potentially damaging revelations.

Even Ms Kelly has said she expected more support from colleagues such as Bill O'Reilly on the station. 'I do wish that O'Reilly had defended me more in his interview with Trump. I would have defended him more,' she said in the April cover story of More magazine.